Effects of changes in carrier frequency and modulation rate on unit responses in the inferior colliculus to amplitude-modulated sounds

Abstract

Unit responses in the rat inferior colliculi to amplitude-modulated sounds were investigated. Two parameters of these sounds were varied: the modulation and carrier frequencies. The first ranged from 1–5 to 100 Hz, while the range of the second was determined by the frequency-threshold curve of the neuron. Other parameters of the amplitude-modulated stimulus (depth of modulation, level of carrier intensity) were kept as constant as possible. Characteristics of the unit response are determined by the nature of the combination of the carrier and modulation frequencies used. If the carrier frequency was optimal, the range of reproduction of the modulation frequencies, the number of spikes in the response, the duration of the response, and the corresponding stimulus duration were maximal; the response appeared at an earlier phase of the modulation cycle. A change in carrier frequency from optimal toward both higher and lower frequencies induced a regular change in all characteristics of the response: narrowing of the range of reproduction of the modulation rhythm amounting in some cases to total cessation of response, a decrease in the number of spikes per response, and shortening of the response in some cases as far as the appearance of only the initial response, while the response itself occurred in later phases of the modulation cycle.